The present invention relates to a method for producing microcapsules and, more particularly, to a method for producing microcapsules using aminoaldehyde polycondensate as wall material according to in-situ method.
Microcapsules comprise a wall of uniform thin film which surrounds and confines liquid, solid or gas as a fine particle of 1 .mu.m--several hundreds .mu.m and those containing, for example, colorless or colored dyes, medicines, agricultural agents, perfumes or feeding stuffs have been industrially commercialized.
Of these uses, application to pressure-sensitive copy sheet is most popular. That is, an over sheet comprising a support coated, on back side, with microcapsules containing hydrophobic liquid in which a colorless electron donating dye is dissolved and an under sheet comprising a support coated, on front side, with colorless electron accepting developer are brought into overlying relationship so that both the coatings contact with each other and pressure is applied thereto by writing to rupture the microcapsules to release the encapsulated materials. As a result, the color former and developer contact with each other and a colored material is formed on the surface of the under sheet by chemical reaction.
As mentioned above, a thin film is formed around a material having a certain specific property and thereby it is also possible to confine the specific property. The enclosed material can be released by rupture of the film when it is needed.
The following methods have been known for preparation of microcapsules:
Coacervation method which uses ionic complex of gelatin and anionic protective colloid.
Interfacial polymerization method which utilizes film-forming reaction at interface between internal and external phases.
In-situ method which forms water-insoluble resin film on the surface of oil droplets from external phase (aqueous phase) [cf. Japanese Patent Publication (Kokoku) No. 60-2100, U.S. Pat. No. 4,100,103 (ethylene-maleic anhydride resin), Japanese Patent Laid-Open (Kokai) No. 54-25277, U.S. Pat. No. 4,233,178 (styrene-maleic anhydride resin), Japanese Patent Laid-Open (Kokai) No. 55-47139 (combination of styrene-maleic anhydride/ vinyl acetate-maleic acid), Japanese Patent Laid-Open (Kokai) No. 56-51238, U.S. Pat. No. 4,533,599 (partial esterified product of styrene-maleic anhydride)].
The above encapsulation methods can afford microcapsules having dense film superior in protection of enclosed core material and are industrially widely used, but still suffer from many problems in production and quality.
That is, coacervation method has the following problems.
(i) Adjustment of pH, temperature and time for reaction and operation are complicated.
(ii) Since it is difficult to obtain microcapsule slurry of more than 20% in concentration, a large quantity of water must be evaporated when used for pressure-sensitive copy sheet. Therefore, there are problems to be improved in operation speed and energy cost.
(iii) The film-forming wall materials are natural products and have a great variability in quality and price.
(iv) The microcapsules have tendency of rotting and agglomeration and cannot be stored for a long time.
These problems in coacervation method have been solved to some extent in the interfacial polymerization method. However, the interfacial polymerization method is not suitable for encapsulation of unstable materials or heat modifiable materials because highly reactive film-forming material is allowed to reacted (at relatively high temperatures).
Besides, problems in solvent resistance and water resistance remain unsolved.
In in-situ preparation method, encapsulation with various amino resins has been proposed and this is now industrially widely applied, but it still suffers from the following problems.
(i) Since water-soluble polymer material which emulsifies hydrophobic liquid as fine droplets has relatively high viscosity, the resulting microcapsule dispersion has naturally high viscosity and thus, it is difficult to obtain a microcapsule slurry having a solid content of at least 50% with good fluidity.
(ii) On the other hand, when water-soluble polymer material of low viscosity or water-soluble polymer material reduced in viscosity by dilution with a suitable solvent is used as an emulsifier, emulsion stability of hydrophobic liquid is reduced, resulting in cohesion or flocculation of the hydrophobic liquid per se.
(iii) In order to obtain physically and chemically high film strength and stability, it is necessary to employ high temperature reaction conditions or to introduce a large amount of film-forming material. When such encapsulation method susceptible to variation of conditions is employed, defective products are apt to be produced in industrial production due to a slight error in setting of conditions or unexpectable change in conditions and scope for industrial application of the products is narrowed.